Abstract: Apparatus for tempering drilling mud preparatory to testing the mud for petroleum content comprising a plurality of cylinders with heat-sinks about their exteriors, resistance heaters in the heat-sinks and temperature control means related to the heaters and heat-sinks, pistons engaged in the cylinders and shiftable longitudinally therein to draw mud into and drive mud out of the cylinders, valve controlled flow means connected with and between the cylinders and upstream and downstream sections of a mud conducting flow line and operable to conduct mud from the upstream section into the cylinders and from the cylinders to the downstream sections and operable by pressure differentials in the cylinders resulting from the movement of the pistons therein, drive means to move the pistons and means controlling operation of the drive means and responsive to the temperature of mud in the cylinders, the pistons operate to wipe and displace mud caked on the walls of the cylinders to be recombined with fluid mud, whereby